1. Field of the Invention
The present invention relates to a ceramic capacitor, and more particularly, it relates to a highly reliable monolithic ceramic capacitor, which can prevent deterioration of insulation resistance under a high temperature ambience.
2. Description of the Prior Art
In recent years, light and small ceramic capacitors have been discussed in the art with development of lightweight miniature electronic components. In particular, study has been made on provision of ceramic capacitors with large capacitance which are small in size with thin-film structure.
Improved thin-film ceramic capacitors may be obtained by the following methods:
(1) To form thin-film dielectric ceramic layers by vacuum thin-film forming methods such as sputtering, vacuum evaporation, ion plating and chemical vapor deposition.
(2) To employ fine grain dielectric ceramic materials thereby to reduce thickness of the dielectric ceramic layers.
(3) To provide insulation layers in grain boundaries of thin-film semiconductor ceramic layers thereby to obtain grain boundary barrier layer type dielectric ceramic layers.
(4) To define inner electrodes between a plurality of dielectric ceramic layers obtained by the above methods (1) to (3) respectively, thereby to form monolithic ceramic capacitors with large capacitance.
However, various problems are caused in ceramic capacitors comprising the thin-film dielectric ceramic layers obtained by the aforementioned methods and electrodes formed therein by sputtering, vacuum evaporation, ion plating, chemical vapor deposition or electroless plating for obtaining capacitance. A particularly remarkable problem is deterioration of insulation resistance under a high temperature ambience.
The most significant cause for such a problem is that dielectric ceramics are formed by metal oxide while the electrodes are made of metals. In other words, such combination inevitably causes transfer of oxygen at the contact surfaces between the metal oxide forming the dielectric ceramics and the metals forming the electrodes. Such transfer of oxygen is not observed under the normal temperature, whereas the same is facilitated by increase in temperature.
In, for example, a ceramic capacitor comprising a dielectric ceramic member of titanium dioxide (TiO.sub.2) and electrodes of copper (Cu), titanium dioxide and copper are changed under a high temperature of, e.g., 150.degree. C. as follows: ##STR1##
When transfer of oxygen thus takes place between the dielectric ceramic member and the electrodes, the dielectric constant .epsilon. of the dielectric ceramic member is changed while insulation resistance IR thereof is significantly lowered substantially over two figures.
Such a phenomenon remarkably takes place in thin-film dielectric layers of each of the ceramic capacitors obtained by the aforementioned methods (1) to (4).
Description is now made on Japanese Patent Publication Gazette No. 28145/1970, which discloses technique similar to the present invention.
According to this prior art example, a solution for forming a film of tin oxide is directly sprayed or coated on the surface of a member of lead zirconate titanate ceramics or barium titanate ceramics etc. to form the tin oxide film, on which silver paste is baked. This prior art example is considered to attain adhesion greater than that of conventional silver electrodes, together with soldering adhesion strength.
However, the tin oxide film is formed by spraying or coating a solution of tin tetrachloride (SnCl.sub.4.5H.sub.2 O), antimony trichloride (SbCl.sub.3) and hydrochloric acid (HCl) on the ceramic member heated at about 700.degree. C., and hence chlorine, which belongs to the halogen group, remains in the tin oxide film formed on the surface of the ceramic member. Thus, the remaining chlorine causes corrosion of metal electrodes for external connection. Further, the said solution sprayed or coated on the thin-film ceramic member penetrates into the same to reduce the insulation resistance thereof, in the worst case.
On the other hand, Japanese Patent Laying-open Gazette No. 187925/1982 discloses a ceramic capacitor which comprises a ceramic dielectric substrate provided on its surfaces with metal oxide layers, first metal layers of either nickel or copper formed thereon and second metal layers of gold, platinum, silver, tin or tin alloy formed on the first metal layers. According to this prior art example, the metal oxide films absorb gas components generated from the ceramic dielectric substrate by heat treatment thereby to prevent the first metal layers of nickel or copper from being separated from the ceramic dielectric substrate. Further, the first metal layers are excellent in adhesion strength with respect to the metal oxide layers, while that with respect to the second metal layers formed thereon is sufficiently secured. Thus, electrodes of the ceramic capacitor are not separated even if the same are exposed to heat in a die bonding process.
There is, however, no description about a monolithic ceramic capacitor in the said prior art example.